Double Pane Windows
The idea behind double-pane windows is to arrest the process of convection as much as possible. In winter, for instance, central heating warms the inside of a home to a temperature much more comfortable than outdoor cold temperatures. But when heat escapes through the window, the central heat system has to work harder to maintain a consistent temperature inside the home, which in turn often leads to higher energy costs. Double-pane windows attempt to prevent this heat loss by trapping heat between the two panes before it can escape to the outside.
To increase the effectiveness of double-pane windows, manufacturers sometimes include gases between the two panes. Heavy gases constrict thermal conductivity, so filling the space between the two window panes with nitrogen can inhibit the flow of warm air from one pane to the other. To further decrease heat loss, windows are sometimes constructed using low-e or spectrally selective glass that is often coated with low emissivity (low-e) coatings.
Nitrogen is not the only gas used in double-pane windows. Argon is one-third heavier than nitrogen or dry air, and krypton is twice as heavy as argon; their higher mass means argon and kyrpton have lower thermal conductivity than nitrogen or dry air, so they make for more energy-efficient double pane windows than those built with nitrogen. Argon and krypton double-pane windows, however, tend to be more expensive than those made with nitrogen since argon and krypton exist at much lower concentrations in air than does nitrogen.
In addition to installing double- or triple-pane windows, there are several steps you can take to decrease heat loss and improve the energy efficiency of your windows. Tightly sealing a heavy-duty plastic sheet over your windows and keeping south-facing windows clean to let in winter sunlight will lessen heat loss in cold weather while using light-colored drapes and shades in lieu of dark-colored ones and closing shades on south- and west-facing windows during hot weather are two examples of these techniques.